The invention relates to an apparatus for selectively taking up a layer of pollutant, particularly hydrocarbons, floating on the surface of a sheet of water and is of particular, although non-exclusive, interest in the depollution of oil-contamined ocean areas.
Such apparatuses are disclosed in U.S. Pat. No. 4,391,708 and in International Application No. WO/83 01 799 of the Applicants. U.S. Pat. No. 4,391,708 describes an apparatus which comprises a hull provided with propulsion means for running before the sea. The hull has a central part projecting forwardly with respect to two lateral parts which define, with the central part, ducts leading to separators. The central part has deflector means, such as fins, for creating swirls or vortices whose orientation tends to reduce the divergence of the surface flow lines around the hull.
One of the functions of the deflector means is to achieve surface sweeping of an area of the sea between flow lines which have a much greater separation upstream of the apparatus than they would have had in the absence of the deflector means. Consequently, the pollutant layer is thicker at the intake of the ducts opening into the separators. That thickening effect is however not sufficient to increase the pollutant content of the mixture delivered to the separator to a sufficient extent. For further increasing the thickening effect along the open flow up to the separator means, there is disclosed, in International Patent Application No. WO/83 01 799 and the parent application, a construction for trapping liquid between the lateral parts and the central part which includes a floor having a negative lift profile. The leading edge of that floor protrudes forwardly of the ducts and slows down the flow in a zone upstream of the leading edge. The trapped flow is minimized by decreasing the size of the water layer above the trapping threshold consisting of a floor and correlatively decreasing the flow speed above that threshold for maintaining a flow of fluvial type, which requires a Froude number v/Vgh lower than 1. That approach provides satisfactory results only as long as the variations of the free level above the floor are of limited amplitude and it is difficult to achieve slowing down while completely avoiding local divergence of the surface flow.
It is an object of the invention to provide an improved apparatus in which the pollutant layer is progressively thickened without any substantial divergence of the surface flow lines. For that result, the trapping action is split into two successive steps:
a superficial trapping, or partitioning consisting in "cutting" the free level of the water sheet by stems of the lateral parts, whose front portion has a depth which is lower than that of the central part, whereby a barrier is formed for the pollutant layer, while in-depth communication is retained and water flows occur from one side to the other of the lateral part;
a complete trapping action, which is effective from the leading edge of an immersed floor which connects the central part to each lateral part, far downstream of the stem of the lateral part.
The bottomless channel which is defined by the external wall of the central part and the internal wall of each lateral part has a width which progressively decreases in a zone upwstream of the immersed floor. Then, there occur a progressive thickening of the pollutant layer and simultaneously water removal from that channel, due to an oblique circumventing flow under the lateral part.
If used alone, such an arrangement would not make it possible to avoid a divergence of the surface flow, upstream of each lateral stem and separation of the flow from the hull surfaces.
For that reason, the front part of each lateral wall is provided with vane means which connect the front portion of each lateral part, immediately behind the stem, with an immersed portion of the central part, forwardly of the stem of the lateral part where that stem cuts the water line. The vane is designed for deflecting the flow lines in the deeper water layers downwardly and outwardly (i.e. away from the longitudinal midplane of the hull). The greater portion of that vane from the central part may be shaped as a swept back wing having a constant chord and an invariant profile, for instance NACA 4515. The undersurface of the vane is not flat, but rather spirally wounded over approximately 90.degree.. Then, the underline of each profile constitutes a line of a cylinder parallel to the axis of the cylinder. The flow is then deflected by an amount which depends on angles .alpha. and .beta. which are the angles between the axis of the cylinder with an horizontal plane and with the vertical midplane of the apparatus, respectively. The vane is connected to a downwardly directed extension of the front portion of that lateral part.
It will be appreciated that the vane intended to avoid divergence of the surface flow line has a shape and a function which are fundamentally different from those of the wing disclosed in Internationnal Patent Application No. WO/83 01799. The function of the latter wing is to compensate for the detrimental side effect of a trapping floor whose leading edge projects forwardly of the lateral parts.
The immersed floor for final trapping preferably has a convex lower surface. The leading edge of that floor is preferably substantially horizontal and swept back in a portion which is close to the central part of the hull. Then, it has a lesser swept back angle and exhibits an upward slope. Finally, the leading edge merges with the vertical profile of the lateral part of the hull. That lateral part is curved downwardly for connection with the floor.
That arrangement makes it possible to locate the vane at a substantial depth, since the vane is not used any longer for limiting the flow rate above it. The flow which is trapped is of course greater than in the arrangement described in application No. WO/83 01799, but the rate of flow is progressively decreased due to progressive removal caused by the vane which exhibits a positive lift and by the floor shaped to provide a negative lift. They induce velocities in the deeper part of the trapped streams which have a downwardly directed component and an outwardly directed lateral component. That change of direction results in a by-pass flow under the lateral part of the hull. The rate of flow after final trapping may be a fraction which is of from 20 to 25% of the rate of flow which enters the channel above the vane. As a consequence, the depth of immersion of the vane may be increased without drawback. Since drainage of the deeper layers is carried out after superficial trapping, there is neither pollutant loss, nor lateral overflow beyond the stems of the lateral parts.
Angles .alpha. and .beta. are selected for avoiding substantial divergence upstream of the stem of each lateral part. That implies that .alpha. and/or .beta. have a positive value, which is however low enough for avoiding disturbance of the flow in the layer close to the surface by the vane. .alpha. and .beta. may generally have values of about 24.degree. and 18.degree., respectively.
Roughness of the sea, particularly due to swell and waves, causes an other difficulty which should also be solved. Breakers may invade the trapping channels in countercurrent with the flow, between the central part and the lateral parts of the hull. In the apparatus of application No. WO/83 01799, that problem was solved by locating a spillway or overfall on the flow path between trapping and discharge. That approach requires pumps which are able to operate under quite variable load and power conditions. It is a further object of the invention to provide a construction which does not require such pumps. It is a more specific object to make it possible to use propulsion pumps which operate at constant speed and power under steady conditions.
For that purpose, the flow which has been finally trapped is fractionated into three distince partial flows. Fractionation is achieved by two additional thresholds provided in addition to the first threshold constituted by the front part of the floor.
The second threshold has a substantially horizontal leading edge which is so located that the greater part of the flow (typically 65 to 70% of the average value of the finally trapped flow) circulates under that second threshold. That greater portion is directly delivered to a propulsion pump which operates at constant power and speed under steady conditions. That greater part represents a flow rate which remains substantially constant during a full period of the wave.
The third threshold is so located that an open flow circulates above it which represents some percents only of the trapped flow. The pollutant is included in that partial flow which is directed to separator means which may be as described in application No. WO/83 01799 and the parent application.
The confined flow between the second and third thresholds or sills by-passes the separator means. It is discharged, in the form of a water sheet overflowing a tangential spillway, into the well of a wave compensator, which will be described later. The pollutant-free flow from the separator means is preferably delivered into that same well, over a second spillway.
Relative movement of the sea and the hull, due to swell and waves, results into a periodical variation of the water height and head in the channels defined by the parts of the hull. That variation occurs with a period which is equal to that of the apparent swell. For avoiding reflection of the compression waves which circulate along the channel when the head varies at the entrance of the channel, the overall flow rate which is trapped should increase at a high rate when the head at the rear of the channel increases. The first of the three partial flows is constant and consequently the variation of the trapped flow as a function of the head should be almost instantaneously provided by the other two partial flows. For that result, the pipes or ducts upstream of the spillways which open into the well of the compensator should fulfil two conditions:
the head loss of three water flow under steady conditions should represent a low fraction only of the reference dynamic pressure .rho.V.sup.2 /2, where V designates the speed of advance of the apparatus through the water body and .rho. is the density;
the inertia of the liquid column should be low enough for the pressure difference between the ends of the pipe caused by acceleration of the flow corresponding to a variation of the flow rate remains low as compared with the reference dynamic pressure .rho.V.sup.2 /2.
The two conditions may be fulfilled with pipes having a sufficiently large flow cross-sectional area. The variations of the level and head are then substantially in phase at the two ends of each pipe, at least when the frequency of the swell is within the range which is currently found at sea.
The relation between the flow rate above a spillway and the thickness of the overflowing sheet of water is such that there is no difficulty in obtaining a large value of the derivative dq/dz.sub.0 (q being the rate of flow and z.sub.0 being the head z+V.sup.2 /2g). It is well known that the rate of flow is in direct relation with power 3/2 of the thickness of the overflowing sheet. As a practical rule, it is sufficient that the head downstream of the spillway, that is in the well which receives the variable flow rate circulating between the second and third thresholds, is always low enough for the Froude number above the spillway to be higher than about 0.5.
For the spillway to be effective in avoiding reflection, the variable flow rate which is discharged into the well should be removed while maintaining such a level in the well that there is a a free and continuous overflow above the spillway. That result may sometimes be obtained by connecting the bottom of the well to the water body through a spirally shaped pipe which has a rearwardly directed exhaust nozzle. Then, forward movement of the apparatus is sufficient to cause a flow from the well. However, recovery of kinetic energy in the spiral may be too low for maintaining the water surface at a low enough level. It is consequently preferable to amplify or assist the rotation caused by tangential injection of the overflows with a motor driven pump.
The energy or power which is required from the pump remains moderate as long as the average ejection velocity is substantially lower than the speed of the apparatus. The pump should accomodate a quite variable flow rate and the rotor may even be partially uncovered at times. It should consequently have a rugged construction. It may consist of an axial pump which forces the flow downwardly. The rotor may however also be quite crude in nature, since it is of low power and a high yield is not an essential requirement. For instance a wheel with longitudinal paddles may be used. Such a wheel is of advantage in that there is no risk of cavitation. As a rule, the overall flow rate which is discharged into the well of the swell compensator may vary, during a period of the swell, between 10 and 40% of the average flow rate trapped during a period, while about 75% of the average rate is ejected by the propulsion pump.
The invention will be better understood from the following description of a particular embodiment given by way of example only.